The present invention relates to printed circuits and their fabrication, and more particularly to multi-layer printed circuits and their fabrication.
Multi-layer or multiple layer printed circuits are made up of a number of layers of printed circuit boards or sheets which are attached by lamination, adhesive such as acrylic adhesive, or other appropriate means. Both surfaces of each of the printed circuit boards included in the multi-layer printed circuit usually have printed circuitry on same. Printed circuitry on one layer can be electrically connected to printed circuitry of another layer via a plated-through hole, which is a hole drilled through all layers to be so interconnected are layers which hole then has metal applied to its interior surface so that an electrical connection between circuitry of different layers is thereby produced. Since registration or correlation of features of each of the layers to be bonded is important, often datum holes are provided to accomplish or more readily permit such registration.
Such printed circuit boards included in such multiple layers can be flexible or rigid. For example, a printed circuit can include layers which are flexible and layers which are more rigid. Multi-layer printed circuits, with flexible layer(s) and more rigid layer(s), can be produced having portions which are flexible and portions which are more rigid, when selected portions of the more rigid layer or layers are removed to expose the flexible portion or portions.
It is often advantageous in processing of a printed circuit board to produce printed circuit boards using layers of standard length and width. Where such standard sizes are larger than the resulting manufactured finished printed circuit boards, more than one such printed circuit board can be produced at a time, by performing appropriate layout of the printed circuit boards thereon such that each finished printed circuit occupies a different, separate portion of such a standard-sized sheet.
Printed circuits can be fabricated wherein conductive paths on outer layer(s) are fabricated after such layer(s) are bonded to or attached to the inner or intermediate layer or layers. Such conductive paths on these or other layers can be formed by plating such paths onto a bare substrate, or by etching a metal-clad substrate to remove all but the desired metallization. In either case, the multi-layer composite is thus subjected to chemicals and other stresses that could be injurious to the conductive paths, etc. on the inner or intermediate layers. Those concerned with the fabrication of multiple layer printed circuit devices have long recognized the need for protection from such hazards. The present invention fulfills this need.
Conventional pre-cut panels often become contaminated with processing fluids during such subsequent processing, resulting in undesirable circuit board yields. The open or pre-cut panel also presents difficult lamination problems to the circuit manufacturer.
It is accordingly desirable to keep plating solution and etching solutions out of areas of circuit panels where their presence would mean entrapment of these fluids within the circuit panel structure, leading to subsequent corrosion or reduced electrical insulation within the circuit itself.
Where a partially flexible and partially more rigid multi-layer printed circuit is to be produced, an opening or window is often cut into the outer, more rigid layers along portions of the future boundary between the flexible and more rigid portions of the finished printed circuit. Such openings can be cut before or after bonding of the outer layer or layers to the rest of the multi-layer structure. In order to reduce or control damage to the inner or intermediate layer or layers with metallization paths are fabricated by plating or etching on such outer layer or layers, after bonding such opening(s) are substantially filled with an adhesive-coated Teflon fiberglass filler. In the field of multi-layer printed circuit fabrication, it has been the general practice to employ adhesive-coated Teflon fiberglass, each piece stamped to match the corresponding opening as a tight fit, for protection of the underlying circuitry, thereby exposed, from such hazards during subsequent fabrication. The pressuresensitive adhesive on each such filler faces the corresponding surface of the flexible layer, and holds it in place during fabrication. Although such formed materials have served the purpose, they have not proved entirely satisfactory under all conditions of service for the reasons that considerable difficulty has been experienced in effectiveness of such sealing. Also, the adhesive on the filler also can contaminate the flexible layer(s), and can rip the flexible layer(s) or metallization thereon when the filler is removed. Also, different openings require production of differently shaped fillers. In addition, different device requirements may involve different thicknesses of hardboard, which requires stocking of different thicknesses of Teflon sheets for such fillers.
Furthermore, the boundary between the flexible and more rigid portions of such a multi-layer printed circuit, being a mechanical interface, is a location of stress. Such stress can cause tearing of the flexible portion of the printed circuit, breaking of conductive paths at the boundary are initial delamination of the cuter layer or layers. A liquid epoxy or urethane plastic bead applied at such boundary has been employed to make the transition from flexible to more rigid portions more gradual, and thereby provide strain relief. However, application and curing of the resin for such beads requires additional fabricating steps.
The liquid plastic bead is difficult to apply, difficult to control in dimensions, takes time to cure, and is difficult to store, and so is expensive to use.
In the flexrigid circuit construction the sharp line formed by contour change between rigidizer board and flex cable becomes a point of stress to induce possible tearing of the flex or cracking conductors. To preclude possible failure in this mode, the usual solution is to apply a liquid epoxy or urethane plastic bead at the facial intersection and then cure the resin in place. The resin then acts as the strain relief.
In producing multi-layer printed circuits with both flexible and more rigid portions, one practice has been to make up the flexible layers of circuitry first. One such material for a flexible substrate is Kapton plastic material. Accordingly, it is desirable to eliminate the need for the careful insertion and removal of such filler pieces. As soon as the flexible layers are processed and cover-coated, the outer layers which will act as the stiffener portion or the component mounting portion are also processed, e.g. conductive paths defined and slots routed. The first step in the outer layers is to put in the datums of the panels, print and etch only the inner layers of the panels, and then proceed to provide cutouts in the panels where the portion of flexible circuitry is to show through the flex rigid structure. The rigid panels are cut along the future boundaries between flexible and more rigid portions of the finished product. The double-clad flexible layer in the center, the appropriate acrylic adhesive and the outer stiffener layers which have been provided with openings or windows are laid up in a laminating fixture by being placed over the datum pins. The flexible area exposed portions are then filled by inserting stamped Teflon fiberglass fillers which have been precoated on one side with an adhesive. These fillers are put into the panel in the open areas as an attempt to seal the panel from the subsequent wet processing. The flexible, interior portion of such a printed circuit device, with the conductive paths and other features already fabricated thereon, are then bonded to the more rigid, outer layer or layers using sheets of acrylic adhesive. The composite is then laminated together under heat and pressure, wherein the acrylic adhesive is cured adhering the stiff members to the flexible portions and creating a panel which has a variety of openings in it where the flexible layers show through. After lamination, the panel is then drilled, plasma processed to produce via or interconnect holes, plated at via holes, unplated holes plugged/and subjected to the print and etch procedures common to printed circuit practices to produce conductive paths and other feature, to produce the exterior circuit portions on the stiff members. Holes that do not require plating and if drilled at the same time as all the other holes, must be plugged such as with a rubber or Teflon pin. Otherwise, such holes must be subsequently drilled, as secondary holes, after the etching is completed. After the panels are thus processed, the filler portions are very carefully extracted from the panel assembly because the flexible areas have a tendency to rip easily, so care has to be taken in removing the fillers from such areas. The fillers are therefore carefully removed to minimize or reduce damage or to attempt to prevent damage to the underlying flexible conductive paths and substrate(s). The panels are then taken to a router which uses the panel datums as reference points and routs or cuts out the hardboard section, with considerable care taken in the now flexible areas not to tear the flexible substrate. The printed circuit devices are then cut out of the bonded or laminated sheets or panels.
There is shown in FIG. 1 a prior art multi-layer printed circuit construction including a flexible printed circuit layer 10. Flexible printed circuit 10 includes a flexible substrate 12 such as of polyimide sold under the trademark "KAPTON" by DuPont Company. KAPTON plastic is produced by a polycondensation reaction between an aromatic terabasic acid or an aromatic dianhydride, and an aromatic diamine. Each surface of substrate 12 bears respective appropriate metallic (such as copper) interconnections, conductive paths and/or features 15, 17. Substrate 12, and metallization 15 and 17, are encapsulated between two flexible layers 11 and 13 such as of adhesive-backed Kapton. Flexible layers 11 and 13 can for example each be of Kapton plastic. Metallization 15 and 17 divided on respective surfaces of flexible substrate 12 can for example be of copper, and can each be formed by plating or deposition on a bare substrate, or by etching an already metal-clad substrate, utilizing photolithography. Layers 11, 12 and 13 are bonded together such as by lamination using adhesive. Disposed on either side of the flexible printed circuit 10 are fiberglass printed circuit boards 19 and 21, which act as stiffeners. Disposed on either side of rigid printed circuit board 19 are metallic (such as copper) interconnections, conductive paths and/or other features 23. Disposed on either side of rigid printed circuit board 21 are metallic interconnections, conductive paths and/or other features 25. Stiffeners 19 and 21 can for example be of fiberglass. Metallization 23 and 25 can for example be of copper, and can be respectively formed on stiffeners 19 and 21 by plating, or etching, with photolithography in substantially the same manner that metallization 15 and metallization 17 were formed. Disposed between flexible layer 11 and stiffener 19 is a bonding layer 27 such as of acrylic adhesive. Disposed between flexible layer 13 and stiffener 21 is bonding layer 29. Layers 27 and 29 can for example each by of acrylic adhesive. When the configuration of FIG. 1 is subjected to heat and pressure to cure bonding layers 27 and 29, stiffener 19 is thereby bonded to flexible layer 11, and stiffener 21 is thereby bonded to flexible layer 13.
There is shown in FIG. 2 a prior art printed circuit 31 including flexible portions 33 and more rigid portions 35. Portions 35 are of the construction of FIG. 1. In producing a printed circuit 31 having flexible portions 33 as well as more rigid portions 35, a single sheet of hardboard is utilized as 5 stiffener 19 for one side of all portions 35, and another, single sheet of hardboard is utilized to produce stiffener 21 for the other surface of all regions 35. In order to prevent bonding of such boards to flexible layers 11 and 13 in portions 33, the board sheets are cut away in such areas. However, following bonding of layer 19 to layer 11 and of layer 21 to layer 13, it may be desirable to thereafter form metallization 23, 25 on the exterior of more rigid portions 35. Accordingly, it has been a general practice to fill in such cutouts with an appropriately shaped Teflon insert or filler which is disposed in each such cutout on each side of flexible layers 11 and 13 at portions 33. Such inserts must be separately shaped and inserted for each flexible area 33. Some shapes may be difficult to form or reproduce. Movement of an insert during fabrication of printed circuit 31 can result in faults or defects in device 31. The surface of each insert contacting a flexible layer 11, 13 bears pressure-sensitive adhesive to limit such movement. While during fabrication the various sheets that ultimately become layers 11, 13, 19, 21, 27 and 29 can and generally do extend beyond the edges of the piece being produced and can be registered to datum points (not shown) having corresponding positions on each such sheet, no such registration can be provided of the inserts. Also, leaks can occur between hardboard 19, 21 and the inserts, permitting the chemicals used in processing metallization 23 and 25 to enter the interior of future device 31 at the openings or cutouts. After the fillers are removed, and device 31 is cut out, a bead 37 of curable liquid epoxy or urethane is applied to each boundary of flexible 33 and more rigid 35 portions. The liquid epoxy or urethane is then cured to form bead lines 37. Bead lines 37 provide strain relief at the boundary between flexible portions 33 and rigid portions 35. However, bead lines 37 require additional steps for application and cure.